FIG. 1 is a diagram illustrating a wireless communication system. Referring to FIG. 1, a wireless communication system 100 includes a plurality of base stations 110 and a plurality of mobile stations 120. The wireless communication system 100 can include a homogeneous network or heterogeneous network. In this case, a network where different network entities coexit, such as a macro cell, a femto cell, a pico cell, and a relay station, is designated as the heterogeneous network. The base stations are fixed stations that perform communication with a mobile station. Each of the base stations 110a, 110b, and 110c provides a service to specific local regions 102a, 102b, and 102c. In order to improve system throughput, the specific regions can be divided into a plurality of smaller regions 104a, 104b and 104c. Each of the smaller regions may be designated as a cell, sector or segment. In case of the IEEE 802.16 system, cell identity (Cell_ID or IDCell) is given based on the whole system. On the other hand, sector or segment identity is given based on the specific region where each base station provides a service, and has a value of 0 to 2. The mobile stations 120 can generally be distributed, fixed or moved in the wireless communication system. Each mobile station can perform communication with one or more base stations through an uplink (UL) and a downlink (DL) at a random time. The base station and the mobile station can perform communication with each other by using FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access), CDMA (Code Division Multiple Access), SC-FDMA (Single Carrier-FDMA), MC-FDMA (Multi Carrier-FDMA), OFDMA (Orthogonal Frequency Division Multiple Access) or their combination. Herein, the uplink means a communication link from the mobile station to the base station while the downlink means a communication link from the base station to the mobile station.